class_docstr: PolyVit base model.
code: def build_flax_model(self):
      raise NotImplementedError(
          "Subclasses must implement build_flax_model()."
      )
  
imports: import functools
  from typing import Any, Dict, Optional, Tuple, List
  from absl import logging
  from flax.training import common_utils
  from immutabledict import immutabledict
  import jax.numpy
  import ml_collections
  import numpy
  from scenic.model_lib.base_models import base_model
  from scenic.model_lib.base_models import model_utils
  from scenic.model_lib.base_models.classification_model import classification_metrics_function
  from scenic.model_lib.base_models.encoder_decoder_model import encoder_decoder_metrics_function
  from scenic.model_lib.base_models.multilabel_classification_model import multilabel_classification_metrics_function
  from scenic.projects.vivit import model_utils
label_desc: Use image node_id 5 for calling the PolyVitBaseModel obj's underlying member method code with example usage: obj.build_flax_model() without return types